The present invention relates to a novel die for extruding tubes composed of a plurality of layers, and more particularly to a die comprising a core and thickness adjusting rings joined at their planar seat surfaces to the core or a thickness adjusting ring joined at its planar seat surface to the core and at least one thickness adjusting ring similarly joined to the ring, the die including resin passages each having resin inlets positioned substantially at the same level as the corresponding planar seat surfaces. The die is easy and inexpensive to make and capable of extruding resin into a tube of a plurality of layers free of any stagnation of the resin flow.
Extrusion dies for the manufacture of tubes composed of a plurality of layers are known which comprise a core and thickness adjusting rings joined to the core each at its spherical seat surface and turnable about the axis of the spherical surface to adjust the clearances at inlet portions of resin passages between the core and the adjacent ring and between the rings. Dies of this type are disclosed for example in Japanese Patent Application Disclosures No. 23461/1972 and No. 73463/1973.
However, it is very difficult to form the spherical seat surface with high accuracy, while it is almost impossible to ascertain during the machining of the parts whether the spherical surface being made is fittable to the counterpart with uniform contact. Accordingly, the formation of the spherical seat surface requires a troublesome procedure in which the parts to be joined together are repeatedly fitted and rubbed together to ensure uniform contact therebetween.